1. Field of the Invention
This invention relates to extrudable organosiloxane compositions. More particularly, this invention relates to extrudable organosiloxane compositions that can be cured by a platinum-catalyzed hydrosilation reaction to form elastomers exhibiting superior physical properties, particularly tear strength and compression set value, without sacrificing other desirable properties such as tensile strength and processability of the curable composition.
2. Description of the Prior Art
Curable organosiloxane compositions containing at least one polydiorganosiloxane with at least two silicon-bonded vinyl radicals, an organohydrogensiloxane or organic peroxide curing agent and a silica filler are well known in the art pertaining to silicone elastomers. It is also known to improve the physical properties, particularly tensile and tear strengths, of cured elastomers prepared from these curable compositions by using two or more vinyl-containing polydiorganosiloxanes of different molecular weights or a single vinyl-containing polydiorganosiloxane having a bimodal molecular weight distribution.
British patent no. 1,269,007 that issued to Wada and Ito on Mar. 29, 1972 teaches peroxide curable compositions containing two polydiorganosiloxanes (1 and 2) each of which contains at least 3000 diorganosiloxane units per molecule and a third polydiorganosiloxane containing from 10 to 1000 siloxane units per molecule. Three different vinyl-containing polydiorganosiloxanes are used to achieve high tear strength in the cured elastomer. Polymer 1 contains either no vinyl radicals or from 0.02 to 0.3 mole percent of vinyl radicals at either terminal or non-terminal positions in the molecule, polymer 2 contains from 5 to 20 mole percent of vinyl-containing siloxane units and polymer 3 contains from 5 to 90 percent of vinyl-containing siloxane units. The tensile strength of the cured elastomers are about 9.8 MPa and the maximum tear strength value reported is 45 kilonewtons per meter (kN/m). The curable compositions would have the consistency of a gum.
U.S. Pat. No. 3,671,480, which issued on Jun. 20, 1972 to the same inventors named on the aforementioned British patent, teaches organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction to yield elastomers exhibiting high values of tear strength. These compositions contain two different vinyl-substituted polydiorganosiloxanes. The first of these polymers contains from 0.02 to 0.2 mole percent of vinyl-substituted siloxane units and an average of at least 3000 siloxane units per molecule, which is equivalent to a gum-type polydimethylsiloxane having a viscosity of at least 1000 Pa.s. The vinyl radicals of this polymer can be located on either terminal or non-terminal repeating units. In all of the exemplified compositions the aforementioned first polymer contains non-terminal vinyl radicals. The second polymer contains an average of at least 100 siloxane units per molecule and at least 2 mole percent of non-terminal vinyl-substituted siloxane units. In the exemplified compositions the first polydiorganosiloxane contains an average of from 6,000 to 10,000 repeating units and from 0.1 to 0.15 mole percent of methylvinylsiloxane units per molecule, and the second polydiorganosiloxane contains from 200 to 8000 repeating units per molecule and from 1 to 50 mole percent of methylvinylsiloxane units. The maximum tear strength value reported for a cured elastomer is 59.4 kN/m. and the tensile strength of this sample is 10094 kPa.
U.S. Pat. No. 4,057,596, which issued to Takamizawa et al. on Nov. 8, 1977 discloses anti-sticking coating compositions containing two vinyl-substituted polydiorganosiloxanes, one of which contains vinyl only at the terminal positions and exhibits a viscosity of from 30 to 5,000 centistokes. The second polydiorganosiloxane contains at least 3 vinyl radicals per molecule that can be at terminal or non-terminal positions and exhibits a viscosity of from 10 centistokes to the consistency of a gum. There is nothing in this patent suggesting addition of filler to obtain compositions useful for preparing elastomeric materials other than films.
U.S. Pat. No. 4,539,357, which issued to Bobear on Sep. 3, 1985 discloses peroxide cured organosiloxane compositions containing two different gum-type vinyl-substituted polymers exhibiting viscosities of from 1000 to 200,000 Pa.s. One of these polymers contains from 0.005 to 0.1 mole percent of vinyl-substituted siloxane units and the second contains from 0.5 to 15 mole percent of these units, with the proviso that the relative concentration of the second polymer decreases as its vinyl content increases. The vinyl-containing siloxane units of both polymers can be located at terminal or non-terminal positions within the molecule. Cured elastomers prepared using these compositions exhibit tear strengths in excess of 22 kN/m. The highest tear strength value reported in the examples is 66 kN/m. The tensile strength of this sample is 7341 kPa.
The composition described in the aforementioned Bobear patent have the consistency of a gum, are not readily processable in conventional injection molding equipment and cannot be transported by pumping using conventional equipment.
U.S. Pat. No. 3,884,866, which issued to Jeram and Striker on May 20, 1975 discloses curable organosiloxane compositions containing (A) 100 parts of a high viscosity component containing at least one vinyl-terminated polydiorganosiloxane having a viscosity of from 5,000 to 1,000,000 centipoise (5 to 1,000 Pa.s) at 25 degrees C., and (B) from 5 to 40 parts of a low viscosity component containing at least one polydiorganosiloxane having a viscosity of from 50 to 5,000 centipoise (0.05 to 5 Pa.s) at 25 degrees C. The curable compositions also contain an organohydrogensiloxane and a platinum hydrosilation catalyst. The structural formulae for ingredients (A) and (B) indicate that the non-terminal repeating units of both the high and low viscosity polymers can contain vinyl radicals, however none of the exemplified compositions contain vinyl radicals bonded to non-terminal repeating units.
The highest tear strength value reported for the cured elastomers prepared using these exemplified compositions is 250 pounds per inch (43.8 kN/m) and the highest tensile strength value is 1100 psi (7590 kPa.). The increase in tear strength and tensile strength is achieved at the cost of the hardness of the cured elastomer, as measured on the Shore A durometer scale. The reported durometer value for this sample is 45.
U.S. Pat. No. 4,162,243, which issued to Lee et al. on Jul. 24, 1979 discloses organosiloxane compositions that cure by the same mechanism disclosed in the aforementioned Jeram and Striker patent. Because of differences in the molecular weight distribution of the vinyl-containing polydiorganosiloxane described by Lee et al. and the vinyl-containing polydiorganosiloxanes of Jeram and Striker, cured elastomers prepared using the compositions of Lee et al. exhibit higher durometer hardness values using less silica filler, with a resultant decrease in the viscosity of the curable composition. This is achieved by the presence in the curable composition of Lee et al. of (1) a silica filler with vinyl-containing organosiloxane groups bonded to its surface and (2) a triorganosiloxy-terminated polydimethylsiloxane composition wherein the triorganosiloxy terminal groups are defined as dimethylvinylsiloxy or phenylmethylvinylsiloxy and the composition exhibits a specified molecular weight distribution and dispersity index. The required molecular weight distribution can be achieved using a single polydimethylsiloxane or a mixture of two or more polydimethylsiloxanes.
The triorganosiloxy terminated polydimethylsiloxanes present in the compositions of the aforementioned Lee et al. patent by definition exclude the presence of ethylenically unsaturated hydrocarbon radicals in any non-terminal repeating unit.
The elastomers prepared by curing the compositions exemplified in the Lee et al. patent exhibit considerably lower tear strength values (35 kN/m) than the maximum value of 43.8 kN/m reported by Jeram et al. and the 66 kN/m reported in the aforementioned Bobear patent, however the maximum durometer hardness values of 62 and 65 measured on the Shore A scale, are considerably higher than the values reported by Jeram et al.
Extrudable organosiloxane compositions curable to elastomers exhibiting desirably high values of tear and tensile strengths are described in U.S. Pat. No. 4,753,978, which issued to J. Jensen on Jun. 28, 1988. The inventive feature of the compositions described in this patent is the combination of a first polydiorganosiloxane (A) exhibiting a viscosity of from 20 to 200 Pa.s and containing ethylenically unsaturated hydrocarbon radicals only at the terminal positions with from 5 to 30 weight percent, based on the first polydiorganosiloxane, of a second polydiorganosiloxane (B) wherein from 1 to 5 percent of the non-terminal silicon atoms contain an ethylenically unsaturated hydrocarbon radical.
Depending upon the concentration of ethylenically unsaturated hydrocarbon radicals present in polydiorganosiloxane (B), the tensile strength of cured elastomers prepared using the compositions claimed in the aforementioned Jensen patent range from 7,266 to 10,191 kPa and the tear strength from 38.5 to 52.6 kN/m.
When one attempts to replace polydiorganosiloxane (A) of the Jensen patent that contains vinyl only at the terminal positions with a polymer exhibiting a viscosity less than 20 Pa.s the tear strength of the resultant cured elastomer is substantially below the values reported in this patent.
The present inventors have now discovered how to increase the tear strength of silicone elastomers prepared by curing silica-filled compositions comprising a diorganoalkenylsiloxy terminated polydiorganosiloxane similar to polydiorganosiloxane (A) of the aforementioned Jensen patent, but exhibiting a viscosity below the range claimed in this patent, an organohydrogensiloxane as the curing agent and a platinum group metal-containing hydrosilation catalyst. They have accomplished this by decreasing the relative concentration of polydiorganosiloxane B to a level below Jensen's lower limit of 5 percent, based on the total weight of polydiorganosiloxanes containing ethylenically unsaturated hydrocarbon radicals. From 1 to 5 mole percent of the silicon atoms of polyorganosiloxane B contain ethylenically unsaturated hydrocarbon radicals.